The past decade has seen rapid growth in the field of Micro Electro Mechanical Systems, which is commonly referred to by its acronym "MEMS". As the name implies, MEMS are basically micro systems which incorporate some type of electromechanical transduction to achieve a given function. In this case, "micro" refers to component features of the order micrometers. Examples of MEMS devices include micropumps, micromotors, micro-optical mirrors, etc. A recent review of the state of the art in MEMS is given in "Micromachines on the March," IEEE Spectrum, May 1994, pp. 20-31.
Many of the MEMS devices reported in the literature use electrostatic transduction. Like most electromechanical transducers, electrostatic transducers can be configured either as actuators or as sensors. When configured as actuators, which are of particular relevance to the present application, electrostatic transducers utilize the attraction of opposite charges to produce a force of attraction. For a parallel plate configuration, this force, or pressure P is readily calculable as follows: ##EQU1## where .epsilon..sub.o is the permittivity of air (8.85.times.10.sup.-12 F/m) and E is the electric field. In the case of parallel electrodes, E=V/d, and so the second relation may be used.
There are numerous examples in the literature of MEMS devices which utilize electrostatic actuating forces. See for example: Zengerle, R., et al., 1992, "A Micro Membrane Pump with Electrostatic Actuation," IEEE Micro Electro Mechanical Systems Workshop.; Gabriel K. J. et al., 1992, "Surface Normal Electrostatic/Pneumatic Actuator," IEEE Micro Electro Mechanical Systems Workshop; Bobbio et al., 1993, "Integrated Force Arrays," Proc. of IEEE MEMS 1993 Workshop, pp. 149-154; and, K. Minami et al., 1993, "Fabrication of Distributed Electrostatic Micro Actuator (DEMA)," J. of MEMS, Vol. 2, No. 3, 1993.
Some of the main reasons for choosing electrostatics over other methods of transduction are as follows:
(1) Energy Density: For a given voltage applied between two electrodes, the electric field increases in proportion to the decrease in separation between the electrodes. Since the electrostatic force is proportional to the square of the electric field, a single order of magnitude closer spacing of the electrodes results in two orders of magnitude greater electrostatic force for the same voltage. Cooperating with this, the electric field strength of most gases also increases rapidly with decreasing distance (see for example: H. L. Saums, "Materials for Electrical Insulating and Dielectric Functions," Hayden Book Co., 1973). Thus it is apparent that electrostatic forces scale well for use in MEMS devices. PA1 (2) Efficiency: Electrostatic devices typically have a relatively high efficiency because they do not require the large current densities, and associated high internal resistance losses, associated with magnetic or shape memory alloy based actuators. The efficiency of an electrostatic device is especially good when the relative electrode motion is a substantial fraction of the inter-electrode gap, as is often the case in MEMS devices. PA1 (3) Cost: Unlike most other transducers, in particular piezoelectric and magnetostrictive, electrostatic transducers require only electrodes which hold the opposite electrical charges to bring about the mechanical force. It is typically much less expensive to deposit electrodes only, than to deposit both electrodes and a piezoelectric material (for example) which is then excited by the electrode.
Although electrostatic actuation mechanisms have the desirable features noted above, there are certain instances where efficiency is not so crucial, and in which it may be more advantageous to employ magnetic actuation. One advantage of magnetic actuation is the ability to achieve forces which act over a longer distance, since the force decreases only linearly with microelectrode separation, as opposed to quadratically in the case of electrostatic forces for a fixed current and potential respectively. Also, lower voltages can typically be employed in magnetically driven actuators since their performance is independent of applied voltage, and depends only on current flow. Even if efficiency is not of great concern, it is necessary to pay close attention to the dissipation of heat produced by the resistive power consumption of the microelectrodes carrying the actuating currents.
The field of MEMS appears to have arisen from two factors: curiosity to explore the limits of miniaturization of electromechanical devices (see for example Feynman, R., 1993, "Infinitesimal Machinery," J. MEMS, Vol. 2, No. 1,) and the widespread availability of micromachining equipment used in the manufacture of integrated circuits. Micromachining techniques are now quite advanced, especially with the recent addition of techniques such as LIGA, silicon fusion bonding, etc.; and allow for the construction of a wide range of devices. But, these micromachining techniques are inherently expensive per unit area, even on large volume production scales, so it appears that they may always be confined to applications which have a very high value per unit area of micromachined surface.
Another limitation of current MEMS technology is that the means for allowing relative motion between the electrodes is provided by mechanical linkages, or the bending of thin, highly cantilevered structures. For example, in the device described in the Bobbio et al. paper referenced above, the spacing between the support points which define each "cell" in the array must be reasonably large relative to the thickness of the polyamide/metal structure, due to the relatively high elastic modulus of these materials. In addition to making the design and construction of such devices quite complex, these relatively thin structures are quite fragile and are therefore not well suited for uses where durability is of concern. These and other disadvantages of prior art MEMS technology can be overcome through the use of a new type of MEMS technology called elastomeric microelectromechanical systems ("EMEMS"), as described below.